Process for producing purified synthesis gas from synthesis gas comprising trace amounts of sulphur contaminants with a metal-organic framework

ABSTRACT

The invention provides a process for producing purified synthesis gas from synthesis gas comprising sulphur contaminants in the ppmv range, the process comprising the step of: (a) contacting the synthesis gas comprising sulphur contaminants with solid sorbent comprising a metal organic framework, thereby separating sulphur contaminants from the synthesis gas to obtain purified synthesis gas.

The present application claims priority from European Patent Application07115991.7 filed 10 Sep. 2007.

The invention relates to a process for producing purified synthesis gasfrom synthesis gas comprising trace amounts of sulphur contaminants.

Synthesis gas is rich in carbon monoxide and hydrogen and furtherusually contains sulphur contaminants. Producing purified synthesis gasfrom synthesis gas comprising trace amounts of sulphur contaminantsinvolves removal of trace amounts of sulphur contaminants. Synthesis gasstreams are generally used in catalytic chemical conversion processes.Often, desulphurization of the feedstock used for the preparation ofsynthesis gas is difficult to achieve or incomplete and consequentlyunwanted sulphur contaminants are still present in synthesis gas.Removal of these sulphur compounds to low levels is of considerableimportance, because they may bind irreversibly on catalysts and causesulphur poisening. This results in a deactivated catalyst, whichseverely hampers the catalytic process. Some catalysts are evensensitive to sulphur concentrations as low as 5 to 10 ppbv. To enablethe use of synthesis gas with these catalysts, sulphur contaminants needto be removed even to the ppbv range. Although bulk removal processesenable removal of sulphur contaminants to a certain level, say forexample to levels in the ppmv range, for removal of trace amounts ofsulphur contaminants to very low levels, in the ppbv range, differentmeasures are needed.

Processes for removal of trace amounts of sulphur contaminants from asynthesis gas are known in the art and are generally based on solid bedadsorption processes.

For example, in U.S. Pat. No. 3,441,370 a process is described forremoval of sulphur compounds from gases by passing the gases over a zincoxide adsorbent. Removal of hydrogen sulphide is achieved at ambienttemperatures. At higher temperatures, removal of RSH and COS is alsopossible. The zinc oxide sorbent employed has a surface area of 30 to100 square meters per gram. The process described in U.S. Pat. No.3,441,370 requires the presence of steam and a temperature of above 300°F. (about 149° C.). It would be desirable to have a more flexibleprocess, enabling removal of trace amounts of sulphur at lowertemperatures.

To this end, the invention provides a process for producing purifiedsynthesis gas from synthesis gas comprising sulphur contaminants in theppmv range, the process comprising the step of:

(a) contacting the synthesis gas comprising sulphur contaminants withsolid sorbent comprising a metal organic framework, thereby separatingsulphur contaminants from the synthesis gas to obtain purified synthesisgas.

Solid sorbents comprising a metal organic framework have been employedin the separation of methane from a mixture of gases including methanefrom other components, as described in European Patent ApplicationEP-A-1,674,555. The gas mixtures to be purified described inEP-A-1,674,555 are relatively clean gases and do not contain any sulphurcontaminants. It has now surprisingly been found that metal organicframework material can be used for removal of trace amounts of sulphurcontaminants.

The process enables removal of sulphur contaminants from the ppmv rangeto very low levels, suitably in the ppbv range. Preferably, sulphurcontaminants are removed to a level of 10 ppbv or less, more preferably5 ppbv or less of total sulphur contaminants.

The process according to the invention can be applied to any synthesisgas, which contains sulphur contaminants in the ppmv range.

Typically, synthesis gas is generated from a feedstock such as naturalgas, coal or oil residue in a synthesis generation unit such as a hightemperature reformer, an autothermal reformer or a gasifier. See forexample Maarten van der Burgt et al., “The Shell Middle DistillateSynthesis Process, Petroleum Review April 1990 pp. 204-209”.

In those cases where the amount of sulphur contaminants in the synthesisgas leaving the synthesis gas producing unit, which can be for example agasifier, a reformer or an autothermal reformer, exceeds 10 ppmv, thesulphur amount in the synthesis gas is preferably reduced in a bulksulphur contaminant removal step as described hereinbefore. This resultsin a synthesis gas stream having an amount of sulphur contaminants of upto 10 ppmv.

The process is especially suitable for synthesis gas comprising a totalamount of sulphur contaminants in the range of from 0.1 to 100 ppmv,based on the synthesis gas. In an especially preferred embodiment, theamount of sulphur contaminants, in particular H₂S and COS, in thesynthesis gas is up to 10 ppmv, preferably up to 5 ppmv. If the sulphurcontaminants include H₂S, the amount of H₂S is preferably up to 500 ppbvH₂S, still more preferably up to 300 ppbv H₂S and most preferably up to100 ppbv H₂S, based on the total gas.

Optionally, the process can be preceded by a bulk contaminant removalstep to reduce the level of contaminants to the ppmv range. Suitablebulk contaminant removal steps include the use of one or more solventformulations based on amines or physical solvents.

In one preferred embodiment, the bulk contaminant removal step is aprocess selected from the group of ADIP, Sulfinol, Flexsorb, Purisol,Rectisol and Selexol. These processes are described in Kohl andRiesenfeld, third edition. These processes are at least partly based onthe finding that carbon dioxide and hydrogen sulphide are highly solubleunder pressure in certain solvents, and readily releasable from solutionwhen the pressure is reduced.

In another preferred embodiment, the bulk contaminant removal step is aprocess based on the direct oxidation of H₂S. For example, a redoxprocess in which the H₂S is directly oxidised to elemental Sulphur usingan iron chelate compound while the ferric ions are reduced, followed byregeneration of the ferric ions by oxidation with air. This process isknown as the SulFerox process. Another example is a combination ofscrubbing the feed synthesis gas with an alkali compounds to convert H₂Sto RS⁻, followed by oxidation of RS⁻ using a biological agent. See forexample WO 92/10270.

In yet another preferred embodiment, the bulk contaminant removal stepis a process based on refigirated methanol as a scrubbing solvent. Whenusing refigirated methanol, sulphur levels of 0.1 ppmv can be achieved.The use of refrigerated methanol is especially preferred when thesynthesis gas is synthesis gas.

All the bulk contaminant removal steps mentioned hereinabove enableremoval of sulphur contaminants to levels in the range of from 0.1 to100 ppmv, or even from 0.1 to 10 ppmv.

The sulphur contaminants in the synthesis gas may include hydrogensulpide (H₂S), mercaptans (RSH) and carbonyl sulphide (COS).

For purified synthesis gas, especially purified synthesis gas that isintended to be used in a catalytic chemical conversion, is oftenrequired that the concentration of sulphur contaminants is in the ppbvrange, say below 10 ppbv, sometimes below 5 ppbv or even as low as atmost 1 ppbv, based on the purified synthesis gas. The process accordingto the invention enables the production of purified synthesis gas havingsuch a low concentration of sulphur contaminants, especially hydrogensulphide.

In step (a), the synthesis gas comprising sulphur contaminants iscontacted with a sorbent comprising a metal organic framework toseparate sulphur contaminants from the synthesis gas to obtain purifiedsynthesis gas. Separation of sulphur contaminants can take place byadsorption of sulphur contaminants from the synthesis gas onto thesorbent. Separation of sulphur contaminants may also take place bypassing the sulphur contaminants to the sorbent, while purifiedsynthesis gas stays behind onto or into the sorbent.

The temperature at which step (a) is carried out may vary between wideranges, and is suitably between 0 and 80° C., preferably between 10 and60° C., and more preferably at ambient temperature. Thus, the processcan be carried out at relatively low temperatures. This offersconsiderable energy-savings compared to conventional trace removalprocesses where a higher temperature is needed.

The pressure at which step (a) is carried out is suitably between 1 and150 bara, more preferably between 1 and 100 bara. Thus, the process canbe carried out at high pressures. This offers advantages in the eventthat the synthesis gas comprising sulphur contaminants is already at ahigh pressure.

Preferably, the metal organic framework comprises at least one metal ionand at least one bidentate organic compound, wherein the bidentateorganic compound is bound to the metal ion.

Suitably, the metal ion is an ion of a metal selected from Groups Ia,IIa, IIIa, IVa to VIIIa and Ib to VIb of the Periodic Table of theelements. References to the Periodic Table and groups thereof usedherein refer to the previous IUPAC version of the Periodic Table ofElements such as that described in the 68th Edition of the Handbook ofChemistry and Physics (CRC Press). Among those metals, particularreference is made to Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr,Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd,Hg, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, Sb, and Bi, more preferably toZn, Cu, Ni, Pd, Pt, Ru, Rh and Co. Most preferred metals are Zn and Cu.

Reference herein to a bidentate organic compound is to a compoundcomprising at least one functional group capable to form at least twocoordination bonds with the metal ion. Especially suitable bidentateorganic compounds are compounds selected from the group of —COOH, —CS2H,—NO₂, —B(OH)₂, —SO₃H, —Si(OH)₃, —Ge(OH)₃, —Sn(OH)₃, —Si(SH)₄, —Ge(SH)₄,—Sn(SH)₃, —PO₃H, —AsO₃H, —AsO₄H, —P(SH)₃, —As(SH)₃, —CH(RSH)₂, —C(RSH)₃,—CH(RNH₂)₂, —C(RNH₂)₃, —CH(ROH)₂, —C(ROH)₃, —CH(RCN)₂ and —C(RCN)₃,wherein R is preferably an alkylene group with 1 to 5 carbon atoms or anarylgroup.

In an especially preferred metal organic framework the metal ion is Cu²⁺and the bidentate organic compound is benzenetricarboxylic acid. Such ametal organic framework is known as “HKUST-1” or “Cu-BTC”. For thepreparation of the sorbent comprising a metal organic framework,reference is made to European patent EP-A-1,674,555.

An advantage of using a sorbent comprising a metal organic framework isthat the BET surface area of such a sorbent is considerably higher thanthe BET surface area of for example a zeolite molecular sieve. Suitably,the BET surface area of the sorbent comprising a metal organic frameworkis at least 500 m²/g, preferably at least 1000 m²/g and more preferableat least 2000 m²/g. Reference herein to the BET surface area is to theBET surface area determined using the standard method DIN 66131.

Without wishing to be bound to a particular theory of how the removal ofsulphur contaminants takes place, it is believed that in most cases step(a) results in purified synthesis gas and solid sorbent comprising metalorganic framework loaded with sulphur contaminants.

In general, the process will not be regenerative, as desorption of thesulphur contaminants will be difficult. It will be understood that theprocess is preferably carried out in a continuous mode. Thus,preferably, step (a) is performed using two or more sorbent beds,wherein at least one sorbent bed is in an adsorbing mode while at leastone sorbent bed, comprising spent sorbent enriched with contaminants, isremoved and replaced by a sorbent bed comprising fresh sorbent. This isusually referred to as operating using a “lead-lag” configuration. Inthis configuration, the synthesis gas is directed to a first bed, theso-called lead bed, which is packed with the solid adsorbent comprisinga metal organic framework. The sulphur contaminants are removed from thegas by the adsorbent, and as a consequence the adsorbent will load withsulphur contaminants. When the sulphur contaminants break through thefirst bed they will flow into a second bed, the so-called lag bed, wherefresh adsorbent comprising a metal organic framework will remove thesulphur contaminants. Once the adsorbent in the lead bed is fullyloaded, the lead bed is taken offline and the adsorbent contained in itis replaced. During the time in which the lead bed is taken offline, thesynthesis gas flow is directed to the lag bed.

The synthesis gas stream may be contacted with solid adsorbent eitheronce or a plurality of times, preferably in a serial manner using morethan one guard bed comprising solid adsorbent, so as to continue toreduce the content of sulphur contaminants. Using the same material inmore than one cleaning or guard bed provides additional advantages. Ifone guard bed fails, there is immediate ‘back up’ to maintain guard ofthe catalyst material, which material is generally much more expensivethan guard bed material. This back-up helps in terms of safety as wellas catalyst preserver. It also allows a guard bed to be off-line forother reasons, such as reloading, regeneration, cleaning, servicing oremergencies, whilst the other(s) guard bed is maintained and the overallcatalytic process continues. Using individual guard bed materials fordifferent impurities requires the catalytic process to stop every timeany guard bed material or guard bed unit must be off-line ormalfunctions.

The purified synthesis gas stream comprises pre-dominantly hydrogen andcarbon monoxide and very low levels, in the ppbv range, of sulphurcontaminants. Preferably, the purified synthesis gas comprises levelssulphur contaminants below 0.1 ppmv, more preferably below 10 ppbv andstill more preferably below 5 ppbv, based on the total purifiedsynthesis gas. The purified synthesis gas is very suitable forconversion to chemicals in a catalytic process. Hence, the inventionalso comprises the purified synthesis gas. The purified synthesis gas isespecially suitable for the manufacture of methanol or ethanol, theproduction of aldehydes using the oxo process, the production of glycolsand the production of hydrocarbons.

In a preferred embodiment, the purified synthesis gas stream iscontacted with a suitable hydrocarbon synthesis catalyst to formnormally liquid hydrocarbons in a hydrocarbon synthesis reaction.

Preferably the purified synthesis gas stream prepared by the presentinvention is used in a number of chemical reactions, in particular inFischer-Tropsch reactions or processes. Catalysts for use in the FischerTropsch reaction frequently comprise, as the catalytically activecomponent, a metal from Group VIII of the Periodic Table of Elements.Particular catalytically active metals include ruthenium, iron, cobaltand nickel. Cobalt is a preferred catalytically active metal.

1. A process for producing purified synthesis gas from synthesis gascomprising a total concentration of sulphur contaminants in the range offrom 0.1 to 100 ppmv, based on the synthesis gas, the process comprisingthe step of: contacting the synthesis gas with a solid sorbentcomprising a metal organic framework, said metal organic frameworkcomprising at least one metal ion bound to at least one bidentateorganic compound, thereby separating sulphur contaminants from thesynthesis gas to obtain purified synthesis gas containing less than 10ppbv total sulphur contaminants.
 2. A process according to claim 1,wherein step (a) is carried out at a temperature in the range of from 0to 80° C.
 3. A process according to claim 2, wherein the totalconcentration of sulphur contaminants in the synthesis gas from whichthe purified synthesis gas is produced is in the range of from 0.1 to 10ppmv, based on the synthesis gas.
 4. A process according to claim 3,wherein the purified synthesis gas comprises less than 5 ppbv of totalsulphur contaminants.
 5. A process according to claim 4, wherein themetal ion is an ion of Zn or Cu.
 6. A process according to claim 5,wherein the metal ion is Cu²⁺ and the bidentate organic compound isbenzenetricarboxylic acid.
 7. A process according to claim 6, whereinthe metal organic framework has a BET specific surface area of at least500 m²/g.
 8. A process according to claim 7, wherein the sulphurcontaminants are selected from the group consisting of hydrogensulphide, carbonyl sulphide and mercaptans.
 9. A process according toclaim 8, wherein the process is preceded by a bulk contaminant removalstep.
 10. A process according to claim 7 wherein the metal organicframework has a BET specific surface area of at least 1000 m²/g.
 11. Aprocess according to claim 7, wherein the purified synthesis gascomprises less than 1 ppbv of total sulphur contaminants.
 12. A processaccording to claim 11 wherein the metal organic framework has a BETspecific surface area of at least 2000 m²/g.